1. Field of the Invention
The present invention is related generally to wafer storage containers, and more particularly to an injection molded storage container with protective walls formed at a draft angle, and wafer positioning posts oriented with no draft angle.
2. Description of the Prior Art
Storage and shipping containers for wafers are often injection molded and have walls surrounding the wafers for protection. FIG. 1 is an illustration of an injection molded container 10. Using standard injection molding practices, the inside surface 12 of the container walls 14 of the container must have what is called a draft angle, such as angle 16 between a perpendicular line 18 from the plane 20 of the container base and a line 22 tangent to the wall 14 for the inside surface 12 of the container. This construction leaves the inside diameter 24 at the base plane 20 of the container smaller than the diameter 26 at the top, which is a condition that allows the container to be removed from the mold from which it is formed. A draft angle is also needed for the outside wall surface.
FIG. 2 is a cross-sectional view of the container of FIG. 1, for more clearly showing the draft angle 16 of the inside wall surface, as well as a draft angle 17 for the outside wall surface, and for illustrating the problem with the injection molded construction when the container 10 is used to transport a stack 28 of wafers. The draft angle 16 is clearly exaggerated in FIGS. 1 and 2 for ease of illustration. The problem with the draft angle, is that it causes a gradual increase in the space 30 between the wafers and the wall as the stack is increased in height, and thereby allowing wafer movement. The wafers can be chipped or scratched as a result of movement in the container, and generally the risk of damage to a disk in a container is proportional to the degree of movement in the container.